On 28/07/16 16:53, Robert Haas wrote:
If I understand correctly, we currently deem an update to be non-HOT
whenever any indexed column is updated.  This is because tuple
versions created by HOT updates can later be removed by HOT pruning,
which means that they must not be referenced by index entries.
Otherwise, after HOT pruning removed the tuple, the index entries
would at best be pointing at nothing and at worse be pointing at some
completely unrelated tuple.

But what about index types that do not store TIDs - i.e. BRIN?  If the

I was thinking about this as well when I've seem the Uber post.

indexed column is updated, we can't actually create a Heap Only Tuple
(HOT), because then the index might be wrong.  But we could create a
Heap Mostly Tuple[1].  We'd construct the update chain in the heap
page just as we would for HOT, and set all the same flags.  But then
we'd also insert new index entries for any TID-free indexes, currently
just BRIN.  For BRIN, that would have the effect of updating the
summary data for that page in such a way that it would encompass both
the old and new values.

I thought about adding another am api function to let index decide if update can be HOT or not, but I like you idea more.

  Petr Jelinek                  http://www.2ndQuadrant.com/
  PostgreSQL Development, 24x7 Support, Training & Services

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